Receptor protein tyrosine phosphatase delta (PTPRD) is a novel tumor suppressor that is widely inactivated in glioblastoma multiforme and several other malignancies. The mechanism(s) of action and the oncogenic context in which PTPRD acts remains largely unknown. This proposal focuses on defining the molecular foundations mediating the tumor suppressive function of PTPRD. Genetic analysis demonstrates that PTPRD and CDKN2A, which encodes the p16Ink4A cell cycle regulator, are frequently inactivated. Their concordant patterns of loss within human glioblastomas suggest that PTPRD may cooperate with CDKN2A during tumorigenesis. We will dissect the functional relationship of PTPRD and CDKN2A during tumorigenesis using the RCAS PDGF glioma mouse model and in vitro human astrocyte models. Studies will focus on determining the cooperative effects of PTPRD inactivation alone or in combination with CDKN2A deletion on tumor initiation and progression, cellular transformation, and alterations in signaling pathways important for cancer cell growth. Our preliminary data shows that STAT3 is a substrate of the PTPRD phosphatase, and recent work in our lab suggests that other substrates may exist. In order to define the molecular mechanisms mediating the tumor suppressive function of PTPRD, we propose to comprehensively characterize the substrate profile of PTPRD using a substrate TRAP assay in conjunction with mass spectrometry. The biological significance of the validated substrates will be investigated. With the completion of this proposal we will gain a better understanding of the molecular foundations of the PTPRD tumor suppressor in GBM, and provide insight into possible therapeutic targets and molecular tools for diagnosis.